To conveniently support an electrical wiring device, such as a switch or a receptacle, an electrical lighting fixture, or simply a cover plate, an electrical outlet, or wiring, box is mounted in an opening made in a wall or ceiling and the electrical apparatus or cover plate is fastened to the box by means of two mounting screws. Conventional electrical outlet boxes are stamped from metal and typically have performed internally threaded bores for receiving and threadedly engaging the mounting screws. More recently, these boxes are made from nonmetallic materials, such as thermoplastic compositions, and various spring clips or other extra inserts are used to fasten the mounting screws to the electrical box as the screw passes through a bore formed in each of the box bosses. The longitudinal axis of the bore is substantially perpendicular to the face of the boss. Such bosses include an internal bore which cooperates with the structure of the clip to mount the clip fixedly in the boss. The use of performed threads in the bore is too expensive.
However, the spring clips used for holding mounting screws in nonmetallic boxes have serious disadvantages. For example, the use of clips presents additional inventory which must be accounted for and ultimately increases the cost of manufacturing the electrical box as well as the chance of losing a necessary part. Moreover, the integral bosses sometimes do not adequately hold the mounting screw to the box, and in other cases, the clips may break under screw loading. In addition, these prior art clips do not significantly serve to angularly align the screw with the mounting bore. Thus, the screw may not easily thread straight into the mounting bore when it is rotated. The screwdriver can slide out of the slot in the screw head because there is no means for holding the screw firmly and therefore, it is free to move from side to side.
Examples of these prior art devices regarding outlet boxes as well as other screw retaining assemblies are disclosed in the following U.S. patents: U.S. Pat. Nos. Re. 29,752 to Jaconette, Jr.; 3,263,949 to Conrad; 3,876,821 to Pringle; 3,967,049 to Brandt; 4,025,144 to Thibeault; 4,130,335 to Kinney; 4,295,003 to Borja et al; and 4,355,198 to Gartland, Jr.
Commonly assigned, copending U.S. patent application Ser. No. 616,201, entitled "Electrical Outlet Box With Integral Screw Aligning, Engaging And Retaining Boss" and filed June 1, 1984, to William E. Slater, now U.S. Pat. No. 4,580,689, discloses a nonmetallic box for supporting a device by a screw. The box comprises a boss with a through bore. The through bore has, in sequence from the boss front face, a frustoconical surface, a first cylindrical surface having a diameter slightly larger than the crest diameter of the screw threads, and a second self-tapping cylindrical surface having a diameter slightly smaller than the crest diameter of the screw threads and slightly larger than the root diameter of the screw threads. The subject matter of this copending application is hereby incorporated by reference.
However, the cylindrical self-tapping bore does not function well with the 8-32 and 6-32 machine screws used to secure electrical fixtures in ceiling boxes (e.g., lights, chandeliers, ceiling fans, etc.) and electrical devices in wall boxes (e.g., switches and receptacles), respectively. The relatively fine threads of the machine screws do not self tap well in thermoplastic materials since they "load up" with plastic material, generate relatively high heat from the friction with the plastic material and do not bite and hold well in the plastic material. Additionally, such cylindrical self-tapping bore is difficult to thread, causing fatigue to installers.
U.L. requirements require boxes to have a minimum strip out torque of 12 inch pounds for 6-32 machine screws and 20 inch pounds for 8-32 machine screws. These requirements must be considered along with ease of insertion.
Accordingly, there is a continuing need for improvement in rapidly, easily and economically mounting various types of electrical devices or covers to nonmetallic outlet boxes.